Traditionally, the raw fibre material used in the production of mechanical pulp has been subjected to a two step mechanical treatment process. The first step is known as "chipping" and includes disintegration of the wood. The second step is referred to as "refining" and involves separating the fibres and increasing their flexibility in one or two refiners. Many of the currently used processes for chipping were developed for the production of chemical pulp, before the introduction of mechanical pulp in the pulp and paper industry. In the production of chemical pulp, the fibre is attacked during chemical cooking (defibration) and is susceptible to weakening and shortening at the sites that have been mechanically deformed. As a result, fibre deformation is unwanted during the production of chemical pulp and therefore minimized.
Apparatuses and processes for chipping have tried to achieve chips of uniform size with minimal fibre deformation. For example, U.S. Pat. No. 3,304,970 describes a new way to produce wafers to avoid the deformation encountered in conventional chipping. The damage to the fibres, e.g. bruising which occurs in chipping is thought to reduce the quality of the wood chips.
Other patent publications concerning chips both for production of paper and board, highlight the benefit of splitting the raw wood material in a direction parallel to the longitudinal axis of the fibres as being less severe to the fibres. For example, U.S. Pat. No. 3,407,854 describes it as preferred to produce chips that are not compressed or damaged.
SE 463 295 concerns the adjustment of the cutting tools in chipping, expressly to avoid the risk of forming longitudinal defects, caused by the pressure, in the fibre walls.
While the industry has increased its production of mechanical pulp, the traditional methods of chipping have nevertheless remained largely unchanged. With some variations for different methods of chipping, the thickness of the chip has been on the order of 3 to 7 mm. These chips have been fed into refiners of different construction for exposing and flexibilizing the fibres. The refiners have been developed for, and adjusted to, the type of chip conventionally used in the production of chemical pulp. The energy consumption during refining is considerable. Additionally, the refining step is a very non-selective way of treating the fibres. During more prolonged refining an unwanted fibre shortening takes place. In spite of this, refining has to be performed up to a certain degree to guarantee that the main part of the fibres is exposed and flexibilized to a sufficient extent.
One special quality of the raw fibre material has gained larger importance in mechanical pulping in recent years. This is that the wood fibres, dividable as they are into fibres with thin walls and thick walls, so-called springwood fibres and summerwood fibres, respectively, have very different mechanical properties. The thick-walled fibres in the summerwood are resistant to strong loads in a direction perpendicular to their length. Whereas, the springwood fibres are more easily deformed by these strong loads applied in the same direction. The summerwood fibre can resist loads that are 10 times greater than the loads that the springwood fibre can resist. All non-selective treatment of fibres leads to more or less unwanted results. For example, refining to the extent needed to flexibilize the summerwood fibres, reduces the springwood fibres in a large extent to fines, which leads to inferior dewatering properties of the pulp and thus, weaker paper. With less extensive treatment, the summerwood fibres retain unreduced stiffness, which leads to unevenness in the paper surface and weaker paper.
The length and anal strength of the wood fibres (both tensile strength and compression strength) should be conserved to as large an extent as possible. The transversal strength of the fibre should nevertheless be reduced, making the fibre more easily collapsible. The transversal strength of the fibres can be reduced and the fibres made flatter, i.e. collapsed, by irreversibly deforming them with a cutting tool. A collapsed fibre has a substantially lower bending stiffness. The lower bending stiffness makes the fibre more easily adaptable to surrounding fibres. The number of contact points between the easily adaptable fibres in the paper thus increases and so does the density of the paper. A collapsed fibre is also flatter, which increases the area of its contact surface with other fibres.
Attempts with rolling the chips, i.e. subjecting them to forces perpendicular to the fibre's length direction, in order to reduce the energy consumption in the production of mechanical pulp, has shown little effect. This is due to the fact that the weaker fibres in the springwood are compressed first and function as a deformation buffer protecting the stronger fibres of the summerwood.